This is my first time using Protocols in Objective-C, and I'm running into a trouble: Here's what I've got:
I have a ReportsReceiver.h:
#protocol ReportsReceiver
-(void)receiveData:(NSArray *)theData;
#end
I have a MyController.h:
#interface MyController : UIViewController<ReportsReceiver,UITableViewDelegate,UITableViewDataSource> {
}
#end
I have a MyController.m with the implemented method:
- (void)receiveData:(NSArray *)theData {
NSLog(#"received some data!");
}
And then I have a class AllUtilities.m with the declaration:
Protocol *receiverProtocol;
AllUtilities.m also contains a method to initialize the protocol:
- (void)initProtocol {
receiverProtocol = #protocol(ReportsReceiver);
}
And then later on in AllUtilities.m I make the call:
[receiverProtocol receiveData:anArray];
Which crashes the application with the error:
2011-01-07 11:46:27.503 TestGA[91156:207] *** NSInvocation: warning: object 0x9c28c of class 'Protocol' does not implement methodSignatureForSelector: -- trouble ahead
2011-01-07 11:46:27.504 TestGA[91156:207] *** NSInvocation: warning: object 0x9c28c of class 'Protocol' does not implement doesNotRecognizeSelector: -- abort
How can I fix this? Thanks!!
You should read the part about protocols in the Objective-C guide once more :) I think you don’t really understand how protocols work. This is what you want:
// DataProducer.h
#protocol DataConsumer
- (void) handleData: (NSArray*) data;
#end
#interface DataProducer
#end
// DataProducer.m
#implementation DataProducer
- (void) generateDataAndPassTo: (id <DataConsumer>) consumer
{
NSArray *data = …;
[consumer handleData:data];
}
// SomeController.h
#import "DataProducer.h"
#interface SomeController <DataConsumer>
#end
// SomeController.m
#implementation SomeController
- (void) requestData
{
// The producer is of type DataProducer.
// Where you get it is irrelevant here.
[producer generateDataAndPassTo:self];
}
- (void) handleData: (NSArray*) data
{
NSLog(#"Got data.");
}
#end
A protocol is, in essence, a contract that says, for example, "an object conforming to the ReportsReceiver protocol must implement the receiveData: method".
So, MyController.h promises that receiveData: will be present, and MyController.m fulfills the promise. So far so good.
Now, your receiver variable doesn't care exactly what type of object the receiver is, so long as it conforms to the ReportsReceiver protocol. The way you declare that is:
id<ReportsReceiver> receiver;
...and in your initialization you might say:
receiver = myController;
Then invoke it like:
[receiver receiveData:anArray];
Start with adding the NSObject protocol to your own protocol. The warnings you are getting are methods from NSObject.
#protocol ReportsReceiver <NSObject>
-(void)receiveData:(NSArray *)theData;
#end
When declaring an object that implements a protocol, it should be more like:
id<ReportsReceiver> receiverProtocol;
or
ReceiverClass<ReportsReceiver> *receiverProtocol;
in the case that you create an object (ReceiverClass) that implements the ReportsReceiver protocol.
You assign a class that implements a protocol in the same way you assign any other class:
ReceiverClass<ReportsReceiver> *receiverProtocol;
- (void)initProtocol {
receiverProtocol = [[ReceiverClass alloc]init];
}
The #protocol directive begins declaring a protocol, not casting to one. Check out the docs for how to use them.
Related
I want to invoke a delegate in class method.
The example below obviously does not work, since the delegate is an instance variable that is accessed within a class method. (Error: instance variable 'delegate' accessed in class method)
Is there an alertnative?
My header file:
// MyClass.h
#import <Foundation/Foundation.h>
#protocol MyDelegate <NSObject>
-(void)update;
#end
#interface MyClass : NSObject
{
id<MyDelegate> delegate;
}
#property (nonatomic, retain) id delegate;
+(void)methodThatInvokesDelegate;
#end
My implementation file:
// MyClass.m
#import "MyClass.h"
#implementation MyClass
#synthesize delegate;
+(void)methodThatInvokesDelegate{
[delegate update];
}
#end
Three obvious options:
Singleton
Static variable (i.e., class variable) pointing to the delegate
Use NSNotification's rather than delegates
Since a singleton (and a static variable) can't keep track of the lifecycle of delegates, I think option three would be the cleanest.
I want to know the context, which let you run in that situation. ;-) Anyway:
First: Delegates are set for a specific instance object. Because of this, you can have different delegates for different instances of the same (delegating) class.
Second: A class method runs inside a class object of that class. This is an object that is different from every instance object of that class. So there is nothing that can be called "the delegate". You can have 100s of delegates.
Third: Your class object needs a delegate at its own. So you have to add a property delegate to the class object and then use this. (Yes, it is possible to have properties an a class object. I did not write declared property.) If you need further information on how to do this, just comment it. I will add code.
I'm not sure if this will help you, but I have a similar situation where I have a class method used for data loads. In this case, the class instantiates itself (so that the caller doesn't need to) until it is done. (this code was edited somewhat to make it work here)
header file:
#protocol DataLoaderDelegate2 <NSObject>
- (void) dataLoaderSuccess:(NSData *)data loader:(id)theloader;
- (void) dataLoaderFailed:(NSString *)error loader:(id)theloader;
#end
#interface DataLoader2 : NSObject {
NSURLConnection *conn;
NSMutableData *receivedData;
NSFileHandle *fileHandle;
id <DataLoaderDelegate2> delegate;
}
#property (nonatomic, assign) id<DataLoaderDelegate2>delegate;
Call to start the process - the call to initWithRequest passes "self" along.
+ (DataLoader2 *)loadWithURLRequest:(NSURLRequest *)req delegate:(id)_delegate
{
DataLoader2 *dl = [[DataLoader2 alloc] init];
[dl setDelegate:_delegate];
conn = [[NSURLConnection alloc] initWithRequest:req delegate:self];
return dl;
}
When the data is done loading, it cleans up with something like
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
if ([delegate respondsToSelector:#selector(dataLoaderSuccess:loader:)])
[delegate dataLoaderSuccess:(fileHandle)?(id)fileHandle:(id)receivedData loader:self];
[self autorelease];
}
I am working in iPhone app with 5 screens. I want to refresh the values in the screen 4th in UITabBarController. I have added #protocol in AppDelegate but it is not calling. This is the first time am using #protocol could you please help me to solve this issue,
In AppDelegate.h
#protocol ReloadViewControllerDelegate <NSObject>
-(void) refreshViewController:(NSString *)result;
#end
id refreshViewControllerDelegate;
#property (nonatomic, retain) id refreshViewControllerDelegate;
and i have synthesized.
In AppDelegare.m
#synthesize refreshViewControllerDelegate;
if ([refreshViewControllerDelegate conformsToProtocol:#protocol(ReloadViewControllerDelegate)])
{
[refreshViewControllerDelegate performSelectorOnMainThread:#selector(refreshViewController:) withObject:#"YES" waitUntilDone:NO];
}// Control not come inside of if Condition.... From here i want to update the fourthViewController..
But control not go inside of the if condition. Could you please guide me where am doing wrong?
In my 4th ViewController.h
#import "AppDelegate"
#interface fourthViewController : UIViewController <ReloadViewControllerDelegate>
In my 4th ViewController.m
-(void) refreshViewController:(NSString *)result
{
NSLog(#"Result : %#", result);
}
Can anyone please help me to do this? Thanks in advance.
You need to declare your delegate like this:
#property (nonatomic, weak) IBOutlet id<ReloadViewControllerDelegate> delegate;
The id will work, but by using the <>, you can make sure that the delegate you assign is actually implementing the protocol, you might still have to make sure it responds to selector but that is only if some methods are declared as
#optional
make sure you synthesize it and most important make sure you set it, and it is not nil.
You're getting a warning because you are typing your delegate as an id. An id is a generic type, which means the compiler has no idea of what methods or properties might be available. In order to remove your warning, declare your delegate to be an NSObject:
#property (nonatomic, retain) NSObject <ReloadViewControllerDelegate> *refreshViewControllerDelegate;
By declaring as an NSObject, the compiler now knows about all the methods NSObject has, which will then allow you to call:
-performSelectorOnMainThread:withObject:waitUntilDone:
on your delegate without warnings. Good luck!
Try this:
#protocol ReloadViewControllerDelegate <NSObject>
-(void) refreshViewController:(NSString *)result;
#end
#interface AppDelegate : UIResponder <UIApplicationDelegate>
#property (strong, nonatomic) UIWindow *window;
#property (weak) id <ReloadViewControllerDelegate>refreshViewControllerDelegate;
#end
In AppDelegate.m
#implementation AppDelegate
#synthesize window, refreshViewControllerDelegate;
...
here Tab4ViewController is name of class.
if ([Tab4ViewController conformsToProtocol:#protocol(ReloadViewControllerDelegate)])
{
[refreshViewControllerDelegate performSelectorOnMainThread:#selector(refreshViewController:) withObject:#"YES" waitUntilDone:NO];
}
...
#end
#import "AppDelegate.h"
#interface Tab4ViewController<ReloadViewControllerDelegate>
...
#end
#implementation Tab4ViewController
...
appDelegate.refreshViewControllerDelegate = self;
...
#end
You are calling this code:
if ([refreshViewControllerDelegate conformsToProtocol:#protocol(ReloadViewControllerDelegate)])
But refreshViewControllerDelegate is this:
id refreshViewControllerDelegate;
conformsToProtocol checks to see if the object declares that it conforms to the protocol, which yours does not. If you want to specify conformity to a protocol you need to:
id<ReloadViewControllerDelegate> refreshViewControllerDelegate;
EDIT
OK, on the performSelectorOnMainThread problem... That method is provided in a category for NSThread, and is not declared in the NSObject protocol. So, if you want to call that, then you need to declare your type as NSObject, which conforms to your protocol.
NSObject<ReloadViewControllerDelegate> refreshViewControllerDelegate;
EDIT
OK, it looks like this is not a simple question about using a protocol, but a full tutorial. Since SO isn't the place for such, I'll try to give a brief one...
A protocol is an interface declaration.
#protocol ReloadChatViewControllerDelegate <NSObject>
- (void)refreshViewController:(NSString *)result;
#end
That says there is a new protocol in town, with the name ReloadChatViewControllerDelegate and it also conforms to the NSObject protocol. Any class that adopts the new protocol must provide an implementation of refreshViewController. You can make a protocol method optional, by putting in an #optional section.
#protocol ReloadChatViewControllerDelegate <NSObject>
- (void)refreshViewController:(NSString *)result;
#optional
- (void)optRefresh;
#end
Now, let's leave the adoption of the protocol for later. Say you are writing generic code, and you just want to know if the object you are given conforms to the protocol, and if so, invoke a method on it. Something like...
#interface Bar : NSObject
#property (nonatomic, weak) NSObject<ReloadChatViewControllerDelegate> *refreshViewControllerDelegate;
- (void)blarg;
#end
Now, the Bar class is providing a delegate property, so that it can be give some object that will help it do some work. However, that delegate object must at least be an NSObject, and conform to the ReloadChatViewControllerDelegate protocol.
Now, ObjC (and C) is quite permissive, so you can force an object to be any type you want, but then you deserve the crash you get. Now, when blarg is called, the delegate is notified to do some work.
Since the property type of the delegate already says it conforms to the given protocol, there is no need to check for conformity. We can just call the delegate method. Note that we must see if the object implements any optional protocol methods.
#implementation Bar
#synthesize refreshViewControllerDelegate = _refreshViewControllerDelegate;
- (void)blarg {
// Do something, then invoke the delegate
[self.refreshViewControllerDelegate
performSelectorOnMainThread:#selector(refreshViewController:)
withObject:#"YES"
waitUntilDone:NO];
if ([self.refreshViewControllerDelegate respondsToSelector:#selector(optRefresh)]) {
[self.refreshViewControllerDelegate optRefresh];
}
}
#end
However, if you want to be generic, and accept any object as a delegate (maybe you want to make it optional that the delegate conforms to some given protocol), then you can accept a plain id and then check to see it it conforms. In that case, you could declare your delegate as just an id (or some other type).
#property (nonatomic, weak) id refreshViewControllerDelegate;
Now, in your code, you need to check for conformity.
- (void)blarg {
// Do something, then invoke the delegate
if ([self.refreshViewControllerDelegate
conformsToProtocol:#protocol(ReloadChatViewControllerDelegate)]) {
[self.refreshViewControllerDelegate
performSelectorOnMainThread:#selector(refreshViewController:)
withObject:#"YES" waitUntilDone:NO];
if ([self.refreshViewControllerDelegate
respondsToSelector:#selector(optRefresh)]) {
[self.refreshViewControllerDelegate optRefresh];
}
}
}
OK, now you have a protocol defined, and you have code that calls methods on the protocol. Two caveats.
First, the delegate has to be set to an object. nil will respond false for any method, so it will of course not conform, nor do anything when sent any message.
Second, you have to make sure that your delegate declares conformity to the protocol. Just implementing the methods is not conformity. If a class does not explicitly specify that is conforms to a protocol, then conformsToProtocol will return false, even if it implements the methods of the protocol.
So, let's specify some class that will act as our delegate by conforming to the protocol.
#interface Foo : NSObject<ReloadChatViewControllerDelegate>
- (void)refreshViewController:(NSString *)result;
#end
#implementation Foo
- (void)refreshViewController:(NSString *)result {
NSLog(#"Look, ma, I'm refreshed with %#", result);
}
#end
It conforms to the protocol, provides an implementation for the mandatory method, and omits the optional one.
Now, if you ran this code, you should see that marvelous code in all its splendor.
Foo *foo = [[Foo alloc] init];
Bar *bar = [[Bar alloc] init];
bar.refreshViewControllerDelegate = foo;
[bar blarg];
(I found some questions discussing the idea, but I didn't see a solution for my problem.)
I added this convenience method as a category to NSObject. (I've added other methods, so I'm still interested in an answer even if you disagree with this particular convenience method.)
#implementation NSObject (MyCategory)
- (void)performInvocationOnMainThread:(NSInvocation *)invocation waitUntilDone:(BOOL)waitForMainThread;
#end
Then I have a protocol I defined:
#protocol MyDelegateProtocol <NSObject>
- (void)myDelegateProtocolMethod;
#end
Then I declare the delegate as a property of my class that implements said protocol.
#property (nonatomic, assign) id <MyDelegateProtocol> delegate;
But when I try to call the NSObject method I added in my category like so
NSInvocation *invocation = [self.delegate invocationForSelector:#selector(someSelector:withArg:)];
I get this warning
'-performInvocationOnMainThread:waitUntilDone:' not found in protocol(s)
If I cast my delegate as (NSObject *) then I don't get the warning. What am I doing wrong? It didn't seem like I could (or should?) add methods to an existing protocol without creating a "sub protocol" and using it from then on. (Which kind of defeats the point of adding methods to NSObject in mind.)
NSInvocation *invocation = [(NSObject *)self.delegate invocationForSelector:#selector(someSelector:withArg:)];
Your category extends the NSObject class, not the NSObject protocol. While the class now has the method, it's not defined as part of the protocol, hence the warning.
That's why typecasting to the NSObject * pointer type works; you're casting to the NSObject class type, rather than something like id<NSObject> which means an arbitrary Objective-C object that conforms to the NSObject protocol.
You'll have to make an intermediate protocol (or "sub protocol") that extends the NSObject protocol:
#protocol ExtendedNSObject <NSObject>
- (void)performInvocationOnMainThread:(NSInvocation *)invocation waitUntilDone:(BOOL)waitForMainThread;
#end
Then have your delegate protocol extend that one instead:
#protocol MyDelegateProtocol <ExtendedNSObject>
- (void)myDelegateProtocolMethod;
#end
If I'm not wrong, you can keep the existing NSObject (MyCategory) implementation, and they'll play nice together.
when pass/expect this type, qualify it like so:
- (void)race:(NSObject<MyDelegateProtocol>*)arg;
I want to subclass UITextView, and send a new message to the delegate. So, I want to extend the delegate protocol. What's the correct way to do this?
I started out with this:
interface:
#import <Foundation/Foundation.h>
#class MySubClass;
#protocol MySubClassDelegate <UITextViewDelegate>
- (void) MySubClassMessage: (MySubClass *) subclass;
#end
#interface MySubClass : UITextView {
}
#end
implementation:
#import "MySubClass.h"
#implementation MySubClass
- (void) SomeMethod; {
if ([self.delegate respondsToSelector: #selector (MySubClassMessage:)]) {
[self.delegate MySubClassMessage: self];
}
}
#end
however with that I get the warning: '-MySubClassMessage:' not found in protocol(s).
I had one way working where I created my own ivar to store the delegate, then also stored the delegate using [super setDelegate] but that seemed wrong. perhaps it's not.
I know I can just pass id's around and get by, but My goal is to make sure that the compiler checks that any delegate supplied to MySubClass conforms to MySubClassDelegate protocol.
To further clairfy:
#interface MySubClassTester : NSObject {
}
#implementation MySubClassTester
- (void) one {
MySubClass *subclass = [[MySubClass alloc] init];
subclass.delegate = self;
}
#end
will produce the warning: class 'MySubClassTester' does not implement the 'UITextViewDelegate' protocol
I want it to produce the warning about not implementing 'MySubClassDelegate' protocol instead.
The UITextView defines its delegate as
#property(nonatomic, assign) id<UITextViewDelegate> delegate
meaning it conforms to UITextViewDelegate, and that's what compiler checks. If you want to use the new protocol, you need to redefine delegate to conform to your protocol:
#interface MySubClass : UITextView {
}
#property(nonatomic, assign) id<MySubClassDelegate> delegate
#end
The compiler shouldn't give any more warnings.
[Update by fess]
... With this the compiler will warn that the accessors need to be implemented... [I implemented this:]
-(void) setDelegate:(id<MySubClassDelegate>) delegate {
[super setDelegate: delegate];
}
- (id) delegate {
return [super delegate];
}
"
[My update]
I believe it should work if you only make a #dynamic declaration instead of reimplementing the method, as the implementation is already there:
#dynamic delegate;
For anyone still interested, this can be done quite simply like this (for sake of the example, I subclass UIScrollView):
#protocol MySubclassProtocol <UIScrollViewDelegate>
#required
-(void)myProtocolMethod;
#end
#interface MySubClass : UIScrollView
#property (nonatomic, weak) id <MySubclassProtocol> delegate;
The most important detail here is the part between the <> after your protocol's name which, put in a simple manner, signals you're extending that protocol.
In your implementation, all you need to do then is:
#synthesize delegate;
And you're done.
You need to extend the super protocol:
#protocol MYClassProtocol <SuperClassProtocol>
-(void)foo;
#end
after that DON'T (!!!) create the #property for the delegate otherwise you override the original delegate object, but simply override the method:
- (id<MYClassProtocol>)delegate
{
return (id<MYClassProtocol>)[super delegate];
}
now you can use the delegate in the classic way:
[self.delegate foo];
[self.delegate someSuperClassDelegateMethod];
Given that MySubClassMessage: is optional, you should be able to simple do a simple:
- (void) SomeMethod {
SEL delegateSelector = #selector(MySubClassMessage:);
if ([self.delegate respondsToSelector:delegateSelector]) {
[self.delegate performSelector:delegateSelector withObject:self];
}
}
The complier should still check that the implementing class conforms to your protocol (or at least claim to in the header) and you won't get the error you described.
i'm getting this two messages:
warning: 'MainView' may not respond to '-switchToNoGridView'
Messages without a matching method signature will be assumed to return 'id' and accept '...' as arguments
1st here:
//GridView.m
#import "GridView.h"
#import "MainView.h"
#implementation GridView
-(IBAction)switchToNoGridView {
[mainView switchToNoGridView];
}
#end
2nd here:
warning: 'MainView' may not respond to '-goBack'
Messages without a matching method signature will be assumed to return 'id' and accept '...' as arguments
in this:
//NoGridView.m
#import "NoGridView.h"
#import "MainView.h"
#implementation NoGridView
-(IBAction)goBack {
[mainView goBack];
}
#end
how to avoid these warnings?
Have you declared switchToNoGridView and goBack in the class interface for MainView?
This warning means that the method signatures could not be found in the class of the instance that you are calling the method on; since message dispatch in Objective-C is done at runtime this is allowed, however a warning is shown.
I'm not quite sure from your code, but I think you're encountering one of two errors. Either:
You haven't declared the methods switchToNoGridView or goBack in the MainView class declaration. For Xcode to know that an object responds to a method, you have to include its definition in the class header file, like this:
#interface MainView : ParentObject {
// instance variables
}
// properties
- (void)switchToNoGridView;
- (void)goBack;
#end
This assumes you actually want to declare them as (void), of course - they can have return values, but since you don't do anything with the result of the call in your code, I'm assuming they're void. Or:
You meant to call MainView the class, not mainView the object. Since we can't see your property or instance variable definitions for GridView.h, nor can we see the current method declarations in MainView.h, it's possible that you have a static method +(void)switchToNoGridView and +(void)goBack declared on MainView, but you're calling it on an instance mainView of MainView. For example:
#interface AClass : NSObject { }
+ (void)doSomething;
#end
#implementation AClass
+ (void)doSomething {
NSLog(#"Doing something");
}
#end
#import "AClass.h"
#interface AnotherClass : NSObject {
AClass *aClass;
}
#property(nonatomic,retain) AClass *aClass;
- (void)doSomethingElse;
#end
#implementation AnotherClass
- (void)doSomethingElse {
[aClass doSomething]; // This will break at runtime
[AClass doSomething]; // but this won't
}
#end
Basically, it's possible you've confused class methods with object methods. Either way, you should check your MainView header file for the appropriate method definitions.